test: 11 edge case E2E tests for demo transliteration

Tests boundary conditions beyond the happy path:
- Empty input, single character
- Very long input (1200+ chars)
- Rapid typing (no debounce issues)
- Mixed scripts (Latin + Cyrillic)
- Special characters (newlines, tabs, emoji surrogate pairs)
- Numbers and punctuation passthrough
- System switching preserves input
- Input that produces no transformation
- Character counter updates

Total E2E: 38 tests (27 happy path + 11 edge cases), all passing.

Author: ronaldtse

e2e/demo-edge-cases.spec.ts

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+/**
+ * Edge case E2E tests for the /demo transliteration page.
+ *
+ * Tests boundary conditions, unicode edge cases, performance,
+ * and error recovery that the happy-path tests don't cover.
+ */
+import { test, expect } from "@playwright/test"
+
+test.describe("demo edge cases", () => {
+  test("handles empty input", async ({ page }) => {
+    await page.goto("/demo")
+    await expect(page.locator(".rail-status")).toContainText("ready", { timeout: 20000 })
+
+    const textarea = page.locator("textarea.pane-body")
+    await textarea.fill("")
+    await page.waitForTimeout(300)
+
+    const output = page.locator(".pane-result")
+    const text = await output.textContent()
+    expect(text).toBeTruthy()
+  })
+
+  test("handles single character", async ({ page }) => {
+    await page.goto("/demo")
+    await expect(page.locator(".rail-status")).toContainText("ready", { timeout: 20000 })
+
+    const textarea = page.locator("textarea.pane-body")
+    await textarea.fill("А")
+    await page.waitForTimeout(300)
+
+    const output = page.locator(".pane-result")
+    const text = (await output.textContent())?.trim()
+    expect(text?.length).toBeGreaterThan(0)
+  })
+
+  test("handles very long input (1000+ chars)", async ({ page }) => {
+    await page.goto("/demo")
+    await expect(page.locator(".rail-status")).toContainText("ready", { timeout: 20000 })
+
+    const longText = "Антон ".repeat(200) // ~1200 chars
+    const textarea = page.locator("textarea.pane-body")
+    await textarea.fill(longText)
+    await page.waitForTimeout(500)
+
+    const output = page.locator(".pane-result")
+    const text = (await output.textContent())?.trim()
+    expect(text?.length).toBeGreaterThan(100)
+    expect(text).toContain("Anton")
+  })
+
+  test("handles rapid typing without errors", async ({ page }) => {
+    await page.goto("/demo")
+    await expect(page.locator(".rail-status")).toContainText("ready", { timeout: 20000 })
+
+    const textarea = page.locator("textarea.pane-body")
+    // Type rapidly character by character
+    for (const char of "АнтонМир") {
+      await textarea.type(char, { delay: 10 })
+    }
+    await page.waitForTimeout(500)
+
+    const errorBanner = page.locator(".error-banner")
+    await expect(errorBanner).not.toBeVisible()
+  })
+
+  test("handles mixed scripts", async ({ page }) => {
+    await page.goto("/demo")
+    await expect(page.locator(".rail-status")).toContainText("ready", { timeout: 20000 })
+
+    const textarea = page.locator("textarea.pane-body")
+    await textarea.fill("Hello Антон мир 123")
+    await page.waitForTimeout(300)
+
+    const output = page.locator(".pane-result")
+    const text = (await output.textContent())?.trim()
+    expect(text?.length).toBeGreaterThan(0)
+    // Latin chars should pass through
+    expect(text).toContain("Hello")
+  })
+
+  test("handles special characters (newlines, tabs)", async ({ page }) => {
+    await page.goto("/demo")
+    await expect(page.locator(".rail-status")).toContainText("ready", { timeout: 20000 })
+
+    const textarea = page.locator("textarea.pane-body")
+    await textarea.fill("Антон\nМир\tПривет")
+    await page.waitForTimeout(300)
+
+    const output = page.locator(".pane-result")
+    const text = (await output.textContent())?.trim()
+    expect(text?.length).toBeGreaterThan(0)
+  })
+
+  test("handles numbers and punctuation", async ({ page }) => {
+    await page.goto("/demo")
+    await expect(page.locator(".rail-status")).toContainText("ready", { timeout: 20000 })
+
+    const textarea = page.locator("textarea.pane-body")
+    await textarea.fill("Антон123!@#")
+    await page.waitForTimeout(300)
+
+    const output = page.locator(".pane-result")
+    const text = (await output.textContent())?.trim()
+    expect(text).toContain("123")
+  })
+
+  test("handles unicode surrogate pairs (emoji)", async ({ page }) => {
+    await page.goto("/demo")
+    await expect(page.locator(".rail-status")).toContainText("ready", { timeout: 20000 })
+
+    const textarea = page.locator("textarea.pane-body")
+    await textarea.fill("Антон 🎉 мир")
+    await page.waitForTimeout(300)
+
+    const output = page.locator(".pane-result")
+    const text = (await output.textContent())?.trim()
+    // Emoji should pass through (not crash)
+    expect(text).toBeTruthy()
+  })
+
+  test("system switching preserves input", async ({ page }) => {
+    await page.goto("/demo")
+    await expect(page.locator(".rail-status")).toContainText("ready", { timeout: 20000 })
+
+    const textarea = page.locator("textarea.pane-body")
+    const select = page.locator("select.field-input")
+
+    // Type in Ukrainian
+    await textarea.fill("Антон")
+    await page.waitForTimeout(300)
+
+    // Switch to Russian
+    await select.selectOption("odni-rus-Cyrl-Latn-2015")
+    await page.waitForTimeout(300)
+
+    // Input should be preserved
+    const inputValue = await textarea.inputValue()
+    expect(inputValue).toBe("Антон")
+
+    // Output should change (different system)
+    const output = page.locator(".pane-result")
+    const text = (await output.textContent())?.trim()
+    expect(text?.length).toBeGreaterThan(0)
+  })
+
+  test("handles input that produces no transformation", async ({ page }) => {
+    await page.goto("/demo")
+    await expect(page.locator(".rail-status")).toContainText("ready", { timeout: 20000 })
+
+    const textarea = page.locator("textarea.pane-body")
+    // Pure Latin text — should pass through unchanged for Cyrillic→Latin map
+    await textarea.fill("Hello World")
+    await page.waitForTimeout(300)
+
+    const output = page.locator(".pane-result")
+    const text = (await output.textContent())?.trim()
+    expect(text).toBe("Hello World")
+  })
+
+  test("character counter updates correctly", async ({ page }) => {
+    await page.goto("/demo")
+    await expect(page.locator(".rail-status")).toContainText("ready", { timeout: 20000 })
+
+    const textarea = page.locator("textarea.pane-body")
+    await textarea.fill("Hi")
+    await page.waitForTimeout(300)
+
+    // Look for character count indicator
+    const meta = page.locator(".pane-meta").first()
+    const metaText = await meta.textContent()
+    expect(metaText).toMatch(/\d+/)
+  })
+})